This invention relates to superconducting circuit elements, namely heterostructures such as SNS Josephson junctions and field-effect transistors, having a sandwich structure consisting of at least one layer of high-T.sub.c superconductor material arranged adjacent to a metallic substrate, having an insulating layer interposed between the high T.sub.c material and the metallic substrate. Both the substrate and the insulator consist of materials having structural properties sufficiently similar to those of the superconductor so that mechanical and chemical compatibility is guaranteed.
For device applications of high-T.sub.c superconductors, it is desirable to find electrically conductive materials, in particular metals, which are compatible with the high-T.sub.c superconductor materials. Compatibility in this context means a best possible match of the respective lattice constants, and the absence of deleterious diffusion effects at the interfaces of the materials involved. These materials may be used, for example, in thin film form for SNS heterostructures (where S stands for superconductor, and N stands for normal metal), or they may serve as the bulk substrate in a field-effect transistor among other applications.
European application EP-A-0 409 338 discloses a planar Josephson device of the type where at least one layer of non-superconducting material is arranged between two layers of a superconductor material. In one embodiment described, the superconductor is YBa.sub.2 Cu.sub.3 O.sub.7-.delta. and the non-superconductor is silver sulphate Ag.sub.2 SO.sub.4. The silver sulphate being connected to the superconductor through layers of silver which serve to prevent diffusion of the sulphate into the superconductor.
The existence of an electrical field-effect in copper oxide high-T.sub.c superconductors (although later found to be very small) has already been reported by U. Kabasawa et al. in Japanese Journ. of Appl. Phys. 29 L86, 1990, and EP-A-0 324 044 actually describes a three-terminal field-effect device with a superconducting channel. Electrical fields are used to control the transport properties of channel layers consisting of high-T.sub.c superconductor materials. While this seemed to be a promising approach, growth studies of such devices have shown that in the suggested configuration, the ultrathin superconducting layers readily degrade during deposition of insulator layer and top electrode.
The lattice constant matching problem has been tackled by using doped and undoped versions of the same material. In particular, the substrate, is used as the gate electrode of a field-effect transistor of the MISFET type. The substrate consists of conducting niobium-doped strontium titanate. Onto that substrate, a barrier layer consisting of undoped strontium titanate is epitaxially grown, and onto the latter the current channel layer consisting of the superconducting YBa.sub.2 Cu.sub.3 O.sub.7-.delta. is epitaxially deposited. It has been found (cf. H. Hagesawa et al., Jap. Journ. Appl. Phys., Vol. 28 (1989) L2210) that during film growth a thin insulating surface layer forms on the Nb-doped SrTiO.sub.3, reducing the sensitivity of the device. The formation of this layer is possible due to a diffusion of Nb atoms from the gate/insulator interface into the bulk of the gate electrode.